(a) Field of the Invention
The present invention relates to a luggage caster, and more particularly to a hollow twin-wheel caster structure applied to the bottom of a luggage.
(b) Description of the Prior Art
A conventional caster installed to the bottom of a luggage is a luggage accessory for the user to pull the luggage conveniently. As disclosed in U.S. Pat. No. 5,163,539 and U.S. Pat. No. 1,153,838, the caster is a single-wheel structure. A twin-wheel caster is available on the market, such as disclosed in U.S. Pat. No. 4,706,329, U.S. Pat. No. 4,351,084, U.S. Pat. No. 4,377,883 and U.S. Pat. No. 6,478,623. Compared to the single-wheel structure, the twin-wheel caster provides better loading capability and stability. The twin-wheel caster as taught in the aforesaid patents comprises a wheel seat coupled to the bottom of the luggage, a horizontal shaft passing through the wheel seat, two ball bearings fitted on two ends of the horizontal shaft, and two wheels fitted on the ball bearings. The diameter of the horizontal shaft is small, so the diameter of the ball bearing is also small. Therefore, the number of the balls in the ball bearings is small, which lowers the loading capability of the balls. The caster of the luggage can be damaged easily.
In order to improve the aforesaid twin-wheel caster and provide a pleasing appearance, the diameter of the hollow shaft is increased to support the ball bearings and the balls, so that the diameter of the ball bearing is increased and there are relatively more balls in the ball bearings, as disclosed in U.S. Pat. No. 7,150,070, U.S. Pat. No. 6,839,939, U.S. Pat. No. 7,647,673, U.S. Pat. No. 7,725,986, U.S. Pat. No. 8,312,595, U.S. Pat. No. 8,375,515. The hollow shaft having a larger diameter is made of a plastic material by injection. The hollow shaft can be manufactured quickly to lower the cost and is light in weight. However, its structural strength is weak. Referring to FIG. 1, when the luggage is overloaded, the weight is applied to the hollow shaft 20 from the center of the wheel seat 10 to cause a slight deformation of the hollow shaft 20 (as shown in FIG. 2). When the hollow shaft 20 is deformed to become an oval-like shape, it will press the rolling route of the balls 301 of the ball bearing 30, such that the roller 40 is unable to roll smoothly. Accordingly, the inventor of the present invention has devoted himself based on his many years of practical experiences to solve these problems.